React Native Firebase database Join or other way for Join - firebase

Question

First of all I know NOSQL systems do not have JOIN. But I know there is a way for getting a datas with another table values.
About my app:
This app is basic social network app. People can share photos. These posts saving firebase database 'realtime database' not cloud firestore than it has a timeline page. this page shows shared posts. I need to show posts with publisher information.
Users login with firebase authentication and I have users table called kullanici like this.
I have Posts table like this.
I need to delete displayName in Posts table than get isim in kullanici table.
my needed data is: Post with kullanici->{posts.{postid}.userid}->isim.
my working code is below:
return (dispatch) => {
firebase.database().ref(`/posts`).endAt("tarih").limitToLast(2)
.on('value', snapshot => {
dispatch({type: STUDENT_LIST_DATA_SUCCESS, payload: snapshot.val()});
});
};
my returned data is below:
Object {
"-L9tnfvm6jQiKLc378c6": Object {
"aciklama": "",
"baslik": "Ensar mı Ahmet mi",
"displayName": "HasanRiza",
"image": "https://hasan-riza-uzuner.s3.amazonaws.com/1523535677033.png",
"like": 244,
"tarih": 1523535757133,
"userid": "fD7IfKAhXogFwHtfKYiF7LMtXNp1",
},
"-LYHPJgR4sywTzpcxX7A": Object {
"aciklama": "Ev",
"baslik": "Nasıl",
"displayName": "HasanRiza",
"image": "https://hasan-riza-uzuner.s3.us-east-2.amazonaws.com/1549718296409.png",
"like": 1,
"tarih": 1549718342522,
"userid": "fD7IfKAhXogFwHtfKYiF7LMtXNp1",
},
}

Answer 1

var promises = [];
group_ids.forEach(function (group_id) {
promises.push(firebase.firestore().collection('Group').doc(group_id).get())
});
Promise.all(promises).then(function(docs) {
docs.forEach((groupDoc) => {
group.name = groupDoc._data['name'];
group.city = groupDoc._data['city'];
group.state = groupDoc._data['state'];
groups.push(group);
});
console.log(groups);
});